Mass spectrometer for analysing substances or indicating a small amount of a determined substance



Nov. 6, 1956 N. WARMOLTZ 2,769,911 MASS SPECTROMETER FOR ANALYSINGSUBSTANCES OR INDICATING A SMALL AMOUNT OF A DETERMINED SUBSTANCE FiledApril 23, 1955 ill/l y IN VEN TOR N ICOLAAS WARMOLTZ AGENT Unite StatesPatent MASS SPECTROMETER FOR ANALYSING SUB- STANCES OR INDICATING ASMALL AMOUNT OF A DETERMINED SUBSTANCE Nicolaas Warmoltz, Eindhoven,

Hartford National Bank and ford, Conn., as trustee Netherlands, assignorto Trust Company, Hart- This invention relates to mass spectrometers foranalysing substances or indicating a small amount of a determinedsubstance.

It is known that mass spectrometers are very suitable for separatingparticles of dilterent molecular weight. In this case the molecules of agas are ionised, whereby, as a rule, separation occurs, whereafter theions are subjected, for example, to the combined action of an electricfield and a magnetic field. In the electric field the ions acquire acertain velocity and in the magnetic field they are deflected from theirpath and this more strongly, according as the mass of the ions issmaller and their load is higher. As a rule, said types of massspectrometer is constituted by a semicircular tube, of which oneextremity contains an ionisation'device and the other extremity containsthe members required for separating the ions according to the mass. Saidmembers are constituted inter alia by a screen comprising a narrow slit,oehind which a collector is arranged. In most mass spectrometers ofknown type, the collector is connected to an amplifier, of which theoutput circuit acts upon an indicator instrument, for example, a meteror an oscillograph.

The invention relates more particularly to the device cator system.

In this case it is preferable that as a rule, is plate-shaped and whichity of emitting secondary electrons or coating the collector with suchmaterial.

larger portion of the electrons is used for the actuation of theGeiger-Muller counter.

According to a further ple beryllium, or it velocity,

meter thick, as is known per se.

2,769,911 Patented Nov. 6, 1956 feature of the invention, for thispurpose a magnetic field is produced in the vicinity of the collector,which field is so directed that a large proportion of the emergingelectrons, after traversing the accelerating electric field, strikes thewindow of the Geiger-Muller counter.

In order that the invention may be readily carried into elfect, it willnow be described with reference to the accompanying drawing showing, byway of example, two embodiments thereof.

' The figures only show that portion of the mass spectrograph which isessential to the invention. The ionisation device and the part in whichthe ions are separated are thus omitted.

In Fig. 1, reference numeral of the constituent of a the beam isband-shaped havmg its largest dimension at The ions finally impinge uponthe collector 4, losing thereon their charge. For this purpose thecollector is connected to a point having a potential substantially equalto that of the electrode 3.

When the ions impinge upon the collector 4, they release therefromelectrons 10, which have a comparatively low speed of emergence, viz. ofthe order of a few volts. In order to increase the number of emergingelectrons, the collector 4 may either consist of material having a highcapacity of emitting secondary electrons, for exam may be coated withsuch material.

According to the invention, provision is made of a plurality ofapertured electrodes 5, 6 and 7, which have positive potentials,preferably progressing, such that the released electrons aresufiiciently accelerated, so that their after having passed the lastelectrode 7, has become such that the electrons of a Geiger-Mullercounter 8. Said window may be constituted by a mica plate of somethousandths of a milli- The discharges brought about in theGeiger-Muller counter by the incoming electrons are counted by means ofa device 9. They may be counted individually, but it is also possible tomeasure the mean electron current. A comparatively small amount of ionsstriking the collector per unit-time alreadyprovides an indicati smallconcentration of a determined gas.

The assembly, except the device 9, may be arranged in the glass envelopeor earthed metal envelope 11 for the tube of the mass spectrometer. Ifthe envelope is of metal, the electrode 2 is preferably connected to theenvelope in a conductive manner. The electrode 3 may have a negativepotential of about 15 kilovolts relatively to earth. The potential ofthe electrode 7 may be 15 kilovolts positive or higher relatively to thecollector 4.

Fig. 2 shows a modification of the device for counting secondaryelectrons. Here the electrons emerging from the collector 4 are subjectto the action of a magnetic field and an electric field, of which thelines of force are at right angles to one another. The electrons emergefrom the collector in different directions, the spatial division of thevelocity approximately following a cosine regularity, such as shown inFig. 3. The magnetic lines of force are at right angles to the plane ofthe drawing so as to be substantially parallel to the surface ofcollector 4. The electric field is produced by the electrodes 4, 12 and14. The electrons emerging at different angles traverse cycloidal paths15 between the electrodes 12 and 14, it being possible by suitablechoice of the strength of the magnetic field and of the potentials ofthe electrodes 12 and 14 that the greater portion of the electrons or atleast a very large portion is focused at the window of theGeiger-Miiller counter 8. The'elect-rode 12 is apertured at this area.The electrons may be further accelerated by means of an electrode 13,which may have apositive potential of afew kilovolts-relatively to thesurroundings 16 designates a scree'iiirig' cap for electrode 13, which'is connected in an electrically conductive manner to the electrode 12.v I

Asia rule, the electrode '14 will have apositive potential of somehundreds of volts with respect to electrode 12. The collector 4 may beconnected in a conductive manner to the electrode 12. Each of saidelectrodes preferably has a negative potential which is approximatelyequal to that of the electrode 3, for example from to kilovolts.

Simila'rly' as in the device shown in Fig. 1, the various electrodes andthe counter 8' are arranged in the envelope for the tube of thespectrometer, which envelopeis not shown here. As an alternative, theGeiger-Miiller counter may be arranged under the plate 12 at the area atwhich the electron paths closely approach one another. The devicesdescribed are very suitable as leak detectors, that is to say forfinding leaks in vacuum tubes. In this case the interior of the tubeunder investigation is connected to the interior of the spectrometertube. The tube under examination is surrounded by a determined gas whichmay be indicated by mean's'of the mass spectrometer, even in extremelysmall concentration, if there is a leak" in the tube.

The Geiger-Miiller counter may be substituted by a crystal counter whichis constituted by a crystal which becomes fluorescent or conductingunder the action of electrons and a device for measuring small amountsof light, for example, a photo-sensitive member with amplification or acounter of current pulses.

What is claimed is:

1. In a mass spectrometer having means producing a beam of ions, acollector in the path of said beam adapted to-produce electrons whenstruck by said beam, electrostatic accelerating means disposed in thevicinity of said collector for accelerating said electrons, radiationdetecting and counting means responsive to incident electronsfordetecting the same and disposed to receive said acceleratedelectrons, and indicating means coupled to'said detecting means. i

2. In a spectrometer as claimed in claim 1 wherein the detecting meansis a Geiger-Miller counter,

3. In a spectrometer as claimed in claim 1 wherein the detectingmeansincludes a scintillation counter.

4. In a spectrometer as claimed in claim 1 wherein the collector ismounted in an inclined position with respect to the path of the beam,said collector inclining toward said electrostatic accelerating means.

5. In .a spectrometer as claimed in claim 1 wherein at least-the surfaceof the collector facing said ion-producing means consists of a materialexhibiting good secondary-electron-emitting properties.

6. In a mass spectrometer having means producing a beam of ions, acollector in the path of said beam adapted to produce electrons whenstruck thereby, electrostatic accelerating means including at least oneperforated plate maintained at a positive potential disposed in thevicinity of said collector outside of the path of the ion beam foraccelerating and directing said electrons, a Geiger-Miiller counterpositioned to receive said accelerated electrons and produce electricalsignals corresponding in intensity to that of the electrons, andindicating means coupled to said counter.

7. In a mass spectrometer as claimed in claim 6 wherein means areprovided produc'ing a magnetic field in the vicinity of said collectorfor focussing the electrons onto the counter.

8. In a mass spectrometer as claimed in claim 7 wherein the magneticlines of force are substantially at right angles to theelectrostaticlines whereby the electrons describe a cycloidal path before beingdetected by the counter.

References Cited in the file of this patent UNITED STATES PATENTS2,422,264 Seaman June 17, 1947 2,543,859 Long Mar. 6, 1951 2,599,166Dempster June 3, 1952 2,625,653 Wouter's Jan. 13, 1953 2,642,535Schroeder June 16, 1953 2,664,515 Smith Dec. 29, 1953 2,720,593 Richardset al. Oct. 11, 1955 OTHER REFERENCES I The Detection of SinglePositiveIons, Electrons and Photons by a Secondary Electron Multiplier by JamesAllen, Physical Review, vol. 55, May 15, 1939, pages 966,972,

